In a wireless communication system it is well known that utilizing linear precoding at the transmitter side can improve the performance substantially when multiple antennas are used in the system. Such kind of linear precoding has been implemented in the IEEE 802.16-2005 standard and in the 3GPP Rel-8/9/10 Long Term Evolution (LTE) standard.
To support precoding at the transmitter in Downlink (DL), the receiver, also known as the User Equipment (UE), feeds back Channel State Information (CSI) about the radio channels experienced between the transmitter and receiver antennas. The CSI consists of:
Channel Quality Indicator (CQI)—the channel quality in the form of Signal to Interference and Noise Ratio (SINR) is measured by the receiver based on the received reference signals. To reduce the overhead of reporting the actual value of SINR, a set of Modulation and Coding Schemes (MCSs) is defined and the measured SINR is mapped to a MCS. Hence, only the index of the selected MCS representing the channel quality is reported to the receiver. The relation between the SINR and the selected MCS is that the Block Error Rate (BLER) of a transport block performed by such MCS does not exceed 10% assuming the transmission channel quality is the measured SINR.
A preferred precoding vector or matrix which the UE has determined based on measurement on the multi-antenna channel.
The suggested rank representing the number of transmission layers or streams that the UE has determined based on the measurement on the DL channel.
To reduce feedback overhead related to precoding vector/matrix, quantization is required as to represent the CSI in a finite number of bits. As an example, the 3GPP LTE Rel.8 standard uses a precoding matrix codebook consisting of 64 matrices for different ranks in case of four antenna ports, and the UE feeds back the preferred precoding matrix using 6 information bits instead of actual channel reporting.
In addition, the corresponding transmission rank and CQI based on the selected precoding matrix are reported as well. After receiving the reported precoding matrix, rank and CQI, the eNB will perform the scheduling for DL transmission for the UE. It should be noted that the reported precoding matrix from the UE is only a recommendation, and the eNB can override the reported one and instead use other precoding matrices which are different from the reported one.
The same principle of precoding operation is also applied in LTE Rel-10 Uplink (UL). For UL precoding, the eNB firstly utilizes the Sounding Reference Signal (SRS) transmitted from the UE to estimate the UL channel. Once the UL channel is measured the eNB can estimate SINR corresponding to the UL channel, and map the SINR to a MCS as DL CQI used for link adaptation. Once UL channel is measured, the eNB selects the rank and the corresponding most appropriate precoding matrix from a predefined codebook of 53 matrices, in the case of four antenna ports, which represent measured channel as well. Finally, the selected precoding matrix is signalled to the UE for UL precoding. The difference with DL precoding is that it is mandatory for the UE to utilize the signalled rank and precoding matrix from eNB.
It can be seen in above described procedures that the precoding matrix is selected by the receiver, and then the precoding matrix is reported or signalled to the transmitter to perform precoding. This kind of precoding operation is defined as closed loop precoding, i.e. channel dependent precoding.
It is important to notice that, for closed loop precoding, the selected precoding matrix based on the channel at time t will be used for precoding at time T (T>t) due to the processing time needed for channel measurement, precoding matrix selection and the propagation time for reporting of precoding matrix or signalling. As the channel is time varying, the selected precoding matrix at time t may be obsolete and not match the channel at time T when the channel varies fast in time such as for high mobility cases. Therefore, the MCS determined by the CQI corresponding to the reported precoding matrix at time t might not be suitable for transmission at time T, which results in performance degradation. Hence, in order to achieve closed loop precoding gain, the channel over time should vary slowly, i.e. low mobility.
As explained above, closed loop precoding is suitable for low mobility. At high mobility, the reported or signalled precoding matrix will not match the radio transmission channel because of channel variation and therefore the performance will be degraded due to this mismatch on chosen precoding vector. However, high mobility is a common scenario in real life, e.g. the communication in the train, car, etc. which must be considered.
In LTE Rel-8, an open loop precoding scheme (DL transmission mode 3) is defined for high mobility scenarios. Major differences between closed loop and open loop schemes are that in an open loop scheme the used precoding matrices are predefined, and further there is no Precoding Matrix (PMI) selection. As both eNB and UE know the predefined precoding matrices, the UE only feeds back average CQI and selected rank as CSI information. In the case of open loop precoding this CQI corresponds to an average CQI. Average CQI over the channel is computed by averaging CQI over a set of the predefined precoders known both at UE and eNB side. More precisely, open loop precoding of Rel-8 uses transmit diversity scheme (T×D) when the transmission rank is 1, otherwise, it uses large delay CDD (Cyclic Delay Diversity) precoding.
For large delay CDD, precoding is defined by:
      [                                                      z                              (                0                )                                      ⁡                          (              i              )                                                                                      z                              (                1                )                                      ⁡                          (              i              )                                                            ⋮                                                                z                              (                                  P                  -                  1                                )                                      ⁡                          (              i              )                                            ]    =            W      ⁡              (        i        )              ⁢          D      ⁡              (        i        )              ⁢          U      ⁡              [                                                                              x                                      (                    0                    )                                                  ⁡                                  (                  i                  )                                                                                                                          x                                      (                    1                    )                                                  ⁡                                  (                  i                  )                                                                                        ⋮                                                                                            x                                      (                                          v                      -                      1                                        )                                                  ⁡                                  (                  i                  )                                                                    ]            
Where P denotes the number of antenna ports, v denotes transmission rank or layers, [x(0)(i) x(1)(i) . . . x(v-1)(i)]T represents the block of data vectors with multiple layers and [z(0)(i) z(1)(i) . . . z(P-1)(i)]T is the block of vectors to be mapped on resources on each of the antenna ports. The precoding matrix W is of size P×v and i=0, 1, . . . , M with M being the number of modulation symbol per layer. The matrix D(i) is supporting cyclic delay diversity the matrix U is of size v×v. These matrices are specified and are given for different number of layers v.
In the case of 4 antenna ports, the UE assumes that the eNB cyclically assigns different precoders to different vectors, i.e. [x(0)(i) x(1)(i) . . . x(v-1)(i)]T, on Physical Downlink Shared Data Channel (PDSCH). A different precoder is used every v vectors. Different precoders are selected from a given table in the specification. Due to precoder cycling, several precoding matrices are used in one Resource Block (RB).
The precoding cycling can achieve an average CQI over four different channels because the four precoding matrices represent four different channels. The averaged CQI is more robust to channel variation than that of closed loop precoding.
A drawback of this scheme is that only one set of four precoding matrices is defined for each rank which is not adapted to different scenarios in practical system. For example, different antenna setups including Uniform Linear Array (ULA) and cross-polarization are deployed in practice, which requires different precoding matrices to match the antenna configuration structure. Moreover, different precoding matrices are needed for the case with/without antenna gain imbalance in uplink due to hand griping. Furthermore, the large delay CDD precoding with precoding cycling can not preserve single carrier property and therefore, is not suitable for the case of LTE UL.
Another prior art is related to Coordinated Multipoint (CoMP) transmission. For CoMP, there are multiple sites and a number of antennas are deployed at each site. For DL transmission, several or all the sites are coordinated together to transmit PDSCH to a certain UE. The PDSCH is transmitted over all the available antennas from the coordinated sites by means of precoding. As the distance between each site and a certain UE is different, the path loss is different and subsets of precoding matrices are predefined corresponding to different path losses.
According to the path loss measurement from the UE, the eNB will select the sites for coordination transmission and determine a subset of precoding matrices for precoding operation from the selected coordination sites. After selection, the determined sites and the subset of precoding matrices are signalled to the UE. Then the UE will follow the procedure as described for closed loop above, i.e. selecting the preferred precoding matrix from the signalled subset and reporting it to the eNB.
It can be seen that the precoding procedure of this prior art is almost similar to the closed loop precoding, which is not suitable for high mobility scenarios. Also, the predefined subsets of precoding matrices are for different number of transmit antennas depending on the number of selected coordination sets. For the normal transmission without CoMP, there is only one site and the path loss is almost same for all the antennas deployed at the site, and therefore defining multiple subsets of precoding matrices corresponding to different path loss and different number of antennas is not useful.
Yet another prior art concerns open loop precoding cycling MIMO communications. According to this prior art, there are multiple subsets of precoding matrices defined for open loop MIMO; for each rank, only one subset of precoding matrix is defined. When a transmission rank is determined for data transmission, all the precoding matrices in the defined subset for such rank will be used to perform precoding cycling.